The present invention pertains to fiber optic sensors and, particularly, to fiber optic current sensors.
Fiber optic current sensors work in the principle of the Faraday effect. Current flowing in a wire induces a magnetic field, which, through the Faraday effect rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as EQU I={character pullout}HdL
where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, .DELTA..phi., through which the plane of polarization of light rotates in the presence of a magnetic field is given by EQU .DELTA..phi.=.intg.H.dL
where V is the Verdet constant of the fiber glass. The sensing optical fiber performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus, one has .DELTA..phi.=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current is measured by injecting light with a well-defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, .DELTA..phi. represents the excess phase shift encountered by a circularly polarized light wave propagating in the sensing fiber.
This technology is related to the in-line optical fiber current sensor as disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor James N. Blake and entitled "Fiber Optic Interferometric Circuit and Magnetic Field Sensor", which is incorporated herein by reference. Optical fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake and entitled, "Fiber Optics Apparatus and Method for Accurate Current Sensing", which is incorporated herein by reference.
The in-line and Sagnac type current sensors disclosed in U.S. Pat. No. 5,644,397, cited above may be operated in a closed loop fashion using direct digitization of the output of the preamplifier attached to the photodetector. The closed loop waveform may incorporate a number of different techniques, well known in the art of fiber optic gyroscopes, including dual ramp, serrodyne, and digital phase ramp. These closed loop techniques, when applied to the in-line and Sagnac type current sensor improve the sensitivity and accuracy of the sensor over that obtainable using the simpler open loop demodulation techniques.
A loop closure scheme typically involves digitization of the output of a preamplifier attached to the photodetector output signal from an analog voltage to a digital one, via an analog-to-digital (A/D) converter in loop closure electronics. An A/D converter "samples"" or converts the preamplifier output to a digital signal representative of the preamp signal periodically, typically several times per half modulation cycle of the bias modulation period of the signal from a bias modulation signal generator. The output of the A/D converter is compared between the two half cycles with a signal indicative of current changes. The presence of a current change changes a loop closure waveform signal is applied to a phase modulator via a digital-to-analog (D/A) converter. The loop closure waveform signal used to rebalance the phase (phi) between the counterpropagating optical waves in the sensing loop may be a sawtooth-type signal (a so-called serrodyne ramp), a digital phase step waveform or a dual ramp waveform. All of these are applied asymmetrically to the interferometer loop to take advantage of the time delay in coil and allow a phase difference (equal and opposite to that generated by current) between the waves, to be generated. The sawtooth or serrodyne waveform has a gradual phase slope proportional to the electric current magnitude with a rapid flyback or reset of a multiple of 2 .pi. phase shift in size. The dual ramp waveform alternates between a positive-going phase ramp and a negative-going one with the difference in the magnitude of the up-slope and the down-slope being proportional to the electric current, all ramp types are capable of reversing to indicate a reversal in the direction of the current in the conductor.
However, a need has arisen for a fiber optic current sensor with much improved sensitivity. Certain applications require that small leakage currents be detected in systems carrying large nominal currents. Examples of such applications include detecting leakage currents in underground distribution cables and in battery charging systems. In charging systems, the detection of leakage currents can be used to provide protection against electrocution of the operator. Also, the current sensor may be part of a ground-fault interrupter.